DACTYLOGRAPHY: THE STUDY OF FINGERPRINTS

NOTE: The following section deals with the detection of fingerprints in the field of forensics and how LED Fluorescence can prove of value. Even if you don't plan to set up your own crime lab, if you study documents and have a need to do some fingerprint hunting to find out if that envelope in your draw with writing on the back really did belong to Abraham Lincoln, this section should be of interest.

The use of clear impressions produced by human fingers and palms dates back to the ancient Babylonians, Chinese, aboriginal Indians, and Egyptians. Those early fingerprints were used to certify contracts, decrees, and bills of sale. Though it is highly debatable as to whether the practitioners were aware of the uniqueness of these marks, the records of such use are there. Ancient pottery, too, sometimes bear the unmistakable imprints of their makers— immortal signatures of the artists! Fingerprints were also found useful for characterizing an individual, much as palm reading purports to do. The system of interpreting one's whorls, arches, and loops to cast a fortune is called dactylomancy.

In the early nineteenth century, Thomas Bewick, an engraver, author, and naturalist, made wood engravings of his fingertip patterns and printed them in books he wrote. Judging by the quality of the impressions, Bewick was very familiar with fingerprints. He used them for stamping receipts. Nevertheless, it isn't completely clear that he fully understood the special character of fingerprints. It really wasn't until the mid-nineteenth century that the unquestioned uniqueness and utility of fingerprints came to the fore. It was probably Sir William Herschel (not the astronomer) who first used the technique of recording and examining fingerprints in India where he was employed as a civil servant. He verified the lifetime permanence of dermatoglyphs, as fingerprints were called. In 1880, a brief note appeared in the English journal, Nature, where Henry Faulds described the use of fingerprints left at the scene of a crime as a means of identifying criminals.

Many names figure prominently in the development of fingerprint science. Individuals such as J. E. Purkinje, Sir Francis Galton, Edward Richard Henry, Juan Vucetich, the husband and wife team of H.H. and I.W. Wilder, and many others were instrumental in helping establish dactylography in those early days. Mark Twain, in his book The Tragedy of Pudd'nhead Wilson, helped popularize the technique by his unsurpassed and concise description of the essence of fingerprinting.

The Practice of Detecting Latent Prints

Rarely do offenders leave clear, unmistakable fingerprints at a crime scene. The invisible or barely visible prints are referred to as latent and need to be developed in order to be "lifted" and/or photographed. Forgers may leave telltale prints on documents. Burglars and robbers may leave prints on safes, jewelry boxes, windows, weapons, dead bodies, or furniture. (Sex offenders leave other traces that also need to be detected.) The nature of the surfaces onto which fingerprints are laid is highly variable. Paper documents may be porous or semi-porous. Safes, windows, guns, and knives are usually nonporous though wooden surfaces (such as some knife handles) are porous. Often, what works for one surface will probably be of little use on another. Items with fingerprints could well be exposed to the elements for substantial periods of time and still may be capable of yielding detectable evidence.

Documents dating back to the Civil War have been successfully fingerprinted! If this doesn't strike you as utterly amazing, just try detecting freshly produced fingerprints on sheets of paper sitting on your desk. But what is a fingerprint and what exactly is being detected? The fingertip is covered with a friction-enhancing series of epidermal ridges (rugae) separated by narrow grooves (sulci). Within these papillary ridges are multiple orifices of sweat glands. These eccrine glands leave traces of a clear sweat containing, at least, sodium chloride, amino acids, proteins, urea, and fatty acids. In addition to the ridges and distinctive sweat pores, skin creases are visible, and these augment the fingerprint traces. The well-named dermatoglyphs or skin carvings are truly unique. If clear prints (from fingers or palms) are left at a scene or on a document, identification is virtually guaranteed.

Fingerprint technicians are usually depicted on TV as carefully and meticulously applying dusting powders with soft brushes with the intent of getting traces of powder to adhere to the tacky secretions left by perpetrators. Dark (e.g., carbon, black ferric oxide) powders might be applied on light surfaces and conversely. This is all well and good for reasonably fresh prints, but what about trace prints? There are established techniques for detecting fatty acids (e.g., colloidal silver). For our purposes here, we will concentrate on the fluorescent detection of amino acids on porous and semi-porous surfaces. We will briefly look at some other methods too.

Ninhydrin, its Analogs, and Fluorescence

In 1910, Siegfried Ruhemann, an English chemist, first prepared ninhydrin. Serendipitously, he discovered that ninhydrin stains the amino acids in the sweat of the hand. Ruhemann studied the violet compound produced, subsequently named Ruhemann's Purple. It wasn't until 1954 that Oden and von Hoffsten reported using ninhydrin to develop fingerprints. Ninhydrin is also used to detect amino acids on thin-layer chromatography plates and in the effluent of chromatography columns. It was later discovered that a secondary treatment with zinc chloride helped to improve the resolution of fine detail in ninhydrin-treated fingerprints. Not only that, but the zinc compound combined with the Ruhemann's Purple to form a fluorescent complex that glowed under illumination with certain wavelengths. This complexing also occurs with cadmium and mercury salts.

To treat surfaces with ninhydrin, the chemical is first dissolved in special solvents that will allow spraying or dipping the item. Because of the health hazards involved, any spraying must be performed in a well-ventilated fume hood. Suitable solvents that will not readily wash away the sought-for fingerprints—but that will dissolve ninhydrin—include mixtures of methanol and trichlorotrifluoroethane. Because the latter chemical (a Freon known as CFC-113) is harmful to the ozone layer, a 3M-engineered fluid, HFE-7100, is now used. But ninhydrin/zinc chloride treatment does not always work well and that could translate into undiscovered fingerprints. Sometimes, liquid nitrogen has to be used to intensify the fluorescence. This adds one more complication to detecting the evidence. (An additional consideration when determining the developer/solvent system to use is compatibility with the article being tested. Inks can run and materials may dissolve. Goodbye evidence, Lincoln's Gettysburg Address, career....)

So, chemists set out to develop chemical analogs of ninhydrin that would improve the sensitivity and reliability of amino acid detection. Work at various labs has produced some amazing improvements over standard ninhydrin. Not only do these analogs reliably detect smaller traces of amino acids, and detect them more often, but some of these analogs do not require a follow-up zinc chloride treatment. Despite eliminating the metal-complexing step, these reagents are capable of producing a far more intense fluorescence than is possible with standard formulations. (Having said all that, it's still true that these analogs may also benefit from a zinc chloride treatment, especially in regard to extended life for the stained prints and, possibly, even greater enhancement of fluorescence!) The downside is that some of these analogs are prohibitively expensive.

Illumination for Forensic Fluorescence Detection: Lasers

Now we get to the crux of this section: light sources used to excite fluorescence in latent fingerprint development! When objects such as documents, checks, currency, or weapons are brought to the forensics lab, often the first action is to bathe them in the intense light from a multiwatt argon-ion laser. (Other lasers, such as copper vapor and frequency-doubled Nd:YAG, are finding their way into use, but argon-ion is the current workhorse.) Of course, the narrow output beam is strongly diverged before encountering any specimen because replacing fingerprints with charred holes is considered really bad form.

Intense laser radiation is capable of exciting an induced fluorescence (LIF). This is particularly true with light of a high actinic value, such as blue or green light. Suitably equipped argon-ion lasers produce lots of light at 488 and 514.5 nm. Lasers with that kind of optical power can be used to excite circulating dye solutions to lase continuously. So, it should not be surprising that organic compounds in unstained latent fingerprints may glow by themselves without additional treatment and can then be photographed. At the very least, the laser could reveal traces that warrant further chemical or physical development. Always, special goggles are required to protect the technician's eyes from the intense laser beam and to allow seeing the fluorescent fingerprints, stains, or particles against a brightly illuminated field.

Illumination for Forensic Fluorescence Detection: Alternative (Alternate) Light Sources

Powerful lasers are very expensive and finicky. Many crime labs in smaller cities and rural counties often cannot afford their initial cost, installation, cooling, and power requirements, as well as their upkeep. Besides that, big lasers are very cumbersome and cannot be considered portable, though technicians at one lab in Canada do truck their argon laser around. And so, alternative light sources were developed to allow thorough testing of crime scenes on location. Suspicious items can then be discovered and carted back to the lab. These forensic lights employ high-wattage xenon or quartz-halogen lamps with suitable filters and fiber-optic cables. Of course, special barrier-filter goggles are required—just as with lasers—and eye hazards still exist, as might be expected. I understand that these light systems do work quite well. However, even these non-laser light sources are expensive.

Enter—the 1-watt and 5-watt LEDs. Based on how well the ultrabright LEDs performed in my fluorescence microscopy experiments, I decided to apply these devices to forensics. One could refer to them as alternative, alternative light sources! While single LEDs may not as yet be bright enough for field work, in a laboratory setting they are really good and should be evaluated even by labs with deep pockets and 20-watt argon-ion lasers. Of course, LEDs can be ganged together to increase both illumination and area of coverage to whatever level is required, and this, too, should be tried.

On the next page, I will put some of these LEDs to the test ferreting out fluorescently stained fingerprints and minute, fluorescent particles and fibers.